Eddy current seismometer



April 1955 D. H. CLEWELL 2,706,805

EDDY CURRENT SEISMOMETER Original Filed Feb. 11, 1947 2 Sheets-Sheet lDAYTON/2C CL EWELL IN V EN T OR.

ATTORNEY April 19, 1955 Original Filed Feb. 11, 1947 D. H. CLEWELL EDDYCURRENT SEISMOMETER 2 Sheets-Sheet 2 A TTORNEV United States Patent EDDYCURRENT SEISMOMETER Original application February 11, 1947, Serial No.727,798. Divided and this application November 29, 1949, Serial No.130,010

3 Claims. (Cl. 34017) This invention relates to methods and systemsinvolving the determination of the velocity of a conductor such as ametal strip resiliently mounted as the stationary element in aseismometer for the detection of seismic waves.

This application is a division of co-pending application Serial Number727,798, filed by applicant February 11, 1947, and now abandoned withoutprejudice to this application, for Methods and Systems for MeasuringVelocity of a Conductor.

Generally, in accordance with the invention, there is produced amagnetic field which traverses the moving conductor to induce eddycurrents therein" and the eddy currents are utilized to produce anelectrical effect, specifically, a current or voltage whose magnitudevaries as a function of the velocity of the conductor.

More specifically, and in accordance with one form of the invention, theeddy currents are utilized in effect to vary the magnetic couplingbetween a-primary coil, forming a part of a seismometer, energized froma source of alternating current, and an associated secondary or pick-upcoil whose induced voltage consequently includes a component whosemagnitude varies in accordance with the relative velocity between aresiliently mounted conductor and the coil: preferably a pair of pick-upcoils are used and are so disposed and interconnected that the variablecomponents of their respective voltages are additive whereas the steadyor zero-velocity components thereof are in opposition. The steadycomponent of the output of a single coil pick-up or any residual steadystate component of the joint output of the preferred twocoil pick-up maybe cancelled by opposition thereto of an equal alternating voltagederived from or having the same frequency as the aforesaid supply sourcewhereby thevariable components or the output voltage will beproportional to the relative velocity between the resiliently mountedconductor and a seismometer case supporting the coil.

The invention further resides in methods, systems and devices havingfeatures herein disclosed and cla1med.

For a more detailed understanding of the invention, reference is "madeto the accompanying drawings, 1n which:

Figure l is a perspective view showing the basic constructional featuresand circuit connections of one form of the invention;

Figure 2 is an end view of ure 1; V

Figure 3 is an explanatory figure referred to in discussion of theprinciples of operation of Figure l;

Figure 4 is a schematic wiring diagram referred to In discussion ofFigure l and other figures;

Figure 5, in perspective, illustrates a modified form of core structure;

Figure 6 is an elevational view, partly 1n section, of a seismometerembodying the invention;

Figure 7 is a plan view of parts appearing in Figure 6; and

Figure 8 is a block diagram of a geophone channel using the seismometerof Figs. 6 and 7.

Referring to Figs. 1 and 2, the strip 10 1s generically illustrative ofa conductor, such as a metal ribbon or strip which when resilientlymounted may form, as will hereinafter be described, the steady mass orinertia element of a vibration sensitive device such as a seismometer.The velocity of the strip 10 or the relative motion between strip 10 andthe pole pieces 11 and 12 parts appearing in Figof the pole faces isdependent upon the driving forces or seismic Waves. In this particularvibration sensitive device, the pole pieces 11 and 12 are those of aC-shaped core member 13, having thereon a coil 14 energized from asuitable source 18 of alternating current.

Assuming the conductor 10 is at rest with respect to the pole pieces 11and 12, traverse of the conductor 10 by the alternating magnetic fluxbetween the opposed faces of the pole pieces 11, 12 produces in thestrip 10 eddy currents represented in Figure 3 by the broken-line arrowsA showing the direction of flow at a particular instant; the direction,of course, reversing with each alternation of the field. These currentsflow in a substantially circular path which is symmetrical about theaxis of the magnetic field between the pole faces.

The magnetic fields induce in each of the pick-up coils 15 and 16, Figs.1 and 2, respectively disposed in advance of and beyond the axis of thepole pieces, a constant voltage which depends upon design or operatingconstants including the coupling of these coils to the primary coil 14as determined by their fixed position with respect to the core member13. For reasons which later herein appear, the coils 15 and 16 areconnected in series opposition so that the voltage 21 and 22, Figure 4,induced therein by the broken-line eddy currents A are in opposition andmore or less completely cancel one another.

Assuming the conductor 10 is in motion relative to pole pieces 11, 12,for example moving in the direction of the arrow, Figure 1, there areproduced, by the motion of conductor 10 relative to the magnetic field,two additional sets of eddy currents; one flowing in a pathsubstantially symmetrical with respect to the upper edges and the otherin a path substantially symmetrical with respect to the lower edges ofthe pole pieces. These eddy currents, respectively indicated by thesolid-linearrows B and C of Figure 3, flow in opposite directions withrespect to each other at each instant because the magnetic gradients arein opposite directions. The direction of the eddy currents, of course,reverses for each reversal of the field producing them. The magnitude ofthese eddy currents is substantially proportional to the velocity ofstrip 10 relative to the pole pieces, other things remaining constant.As indicated in Fig ure 3, the upper eddy currents B, symmetrical withrespect to the upper edges of the pole pieces, are additive to thesteady eddy currents represented by the brokenline arrows A whereas thelower group C of eddy currents, symmetrical with respect to the loweredges of the pole pieces, are subtractive from the steady eddy currents.Accordingly, the total voltage induced in the uppermost pick-up coil 15comprises a steady component e1, corresponding to the inductive couplingof the pick-up coil with the eddy currents A, and a variable componentAe additive thereto corresponding with the eddy currents B, with thistotal voltage proportional to the applied alternating magnetic field.The total voltage induced in the lower pick-up coil 16 includes a steadycomponent e2 corresponding with the eddy currents A, and a variablecomponent As in opposition thereto, corresponding with the eddy currentsC, with this total voltizigiedalso proportional to the appliedalternating magnetic When the coils 15 and 16 are connected aspreviously described, the steady components of the voltages induced incoils 15 and 16 mutually cancel whereas the variable components areadditive so that the elfective joint output (ZAe) of the two coils issubstantially proportional to the velocity of the conductor 10 and tothe applied alternating magnetic field, appearing as an alternatingvoltage with amplitude modulation represent ative of the motion of strip10.

Because of difiiculties in manufacture, it is often not feasible oreconomical to attain complete balance or cancellation of the steadycomponents el and e2 of the individual voltages of the pick-up coils 15and 16. Any residual steady component of the joint output of the pick-upcoils may, however, readily be cancelled by opposition thereto. of avoltage e3, Figs. 1 and 4, derived from or having the same frequency asthe source 18 used to energize the coil 14 of the device. The source 19,Fig. 1, of this compensating voltage 23 may be of any known type; asshown in Fig. 4, it may comprise a phase-shifting network 20 includingresistors 21, 21 and reactances 22, 22 arranged in the form of aWheatstone bridge. By using reactances and resistors which are of equalimpedance at the frequency of source 18, a phase shift of any desiredmagnitude from to 180 may be obtained and without change of amplitude bysimultaneously and equally varying either both resistors or bothreactances', for low frequencies, it is more convenient to use variableresistors whose movable elements are ganged for simultaneous adjustment.By such adjustment, the voltage 23 may be adjusted so that it is inphase opposition to the algebraic sum of the steady-component voltagesel and e2 produced respectively by the pick-up coils and 16. To attain avalue of e3 which is numerically equal to the unbalance of the voltagesel and e2, the input terminals of the bridge may be connected to avoltage divider 23 having a contact adjustable to vary the input voltageto the phase-shifter 20. Preferably, and as shown, the phase-shiftingnetwork and the measuring network comprising the pick-up coils and themeasuring, recording or control device 17, are isolated from the supplycircuit by a step-down transformer 24. When only one pick-up coil 15 or16 is utilized, the magnitude of voltage e3 is, of course, selected oradjusted to be suitably greater to effect balance.

In the modification shown in Fig. 5, the pole piece 12 is divided orsplit in direction normal to the direction of relative movement ofconductor 10, forming two legs 12A and 123 about which the pick-up coils15 and 16 are respectively disposed. With this construction, as comparedto that of Fig. 1, the sensitivity is higher and the disturbing effectof extraneous magnetic fields from other nearby equipment is reduced.

In Figs. 6 and 7, there is illustrated a seismometer Y which convertsearth vibrations into electrical signals useful in seismic prospectingsystems in which an explosive charge is detonated and the record of theresulting seismic waves interpreted to determine the depth and contourof subterranean strata. The seismometer or geophone comprises a casing26 suited for disposition on the surface of the ground or in a borehole. The metal ribbon or strip 10A corresponding with strip 10 of Fig.1 is resiliently attached to the casing 26 as by a pair of leaf springs27 suitably connected to the upper and lower ends of the strip. Toincrease the inertia, a weight 28 may be attached to the free end orportion of the resiliently mounted system. The core member 13, theexciting coil 14, and the pick-up coils 15 and 16 form a unit which isrigidly attached to the casing 26 by the bracket 29. The pick-up coils15, 16 may be disposed on legs 12A, 12B of pole piece 12 as in Fig. 5,orlmay be adjacent the face of pole piece 12 as in Fig.

Upon arrival of seismic waves at the seismometer location, the inertiaelement comprising the strip 10A will remain at rest, whereas the casingand the core member 13 will vibrate with respect thereto. In this as inthe modification previously discussed, the relative motion of theconductor and the magnetic field produce in each of the pick-up coils15, 16 a voltage whose magnitude is a function of the velocity of theconductor. The motion of the conductor between the coil 14 on one handand the coils 15 and 16 on the other, modulates the voltage induced inthe pick-up coils 15, 16 so that their output is of the same frequencyas the source 18 amplitude-modulated in accordance with vibrations ofthe conductor.

The output voltage of the seismometer is preferably magnified by anamplifier of suitable number of tuned or filter circuits which are thecarrier frequency; that is, the 18, and whose band width is suitable topass seismic wave frequencies which are generally of the order of fromthirty to eighty cycles per second. The carrier frequency issubstantially higher; for example, 1000 cycles per second. The output ofthe amplifier 30 is rectified as by the demodulator 31 and the seismicfrequency output thereof is impressed upon the galvanometer 32 or othersuitable recording device. Alternatively, the seismometer of Figs. 6 and7 may be used in the measuring circuit of Fig. 4.

It shall be understood the invention is not limited to the particularembodiments disclosed but that changes and modifications may be madewithin the scope of the appended claims.

What is claimed is:

1. A seismometer unit for producing an electrical output representativeof seismic waves comprising a casing, magnetic core structure rigidlyattached to said casing and having pole pieces spaced to provide a gap,an inertia element comprising a metallic strip extending in a planethrough said gap and resiliently supported at its opposite ends fromsaid casing, exciting means for producing an alternating magnetic fluxin said gap, said alternating magnetic field upon relative movement ofsaid strip inducing therein at two spaced regions eddy currents ofinstantaneously opposite directions of flow and affecting thedistribution of said flux in said gap, and a pair of stationary pick-upcoils having their axes spaced in a direction which is normal to theflux traversing said gap, the extent of such spacing in the direction ofmovement of said strip being such that said stationary pick-up coils arerespectively disposed adjacent said two eddy current regions of therelatively movable strip.

2. A seismometer system as in claim 1 in which the pole pieces of saidmagnetic core structure which define said gap have opposed pole faces ofunbroken cross-section and in which the axes of coils are outside theboundaries of said pole faces.

3. A seismometer unit for producing an electrical output representativeof seismic waves comprising a casing, an inertia element including ametallic strip resiliently attached to said casing for vibration bodilywith respect thereto, magnetic core structure rigidly attached to saidcasing and having pole pieces whose opposed faces define a gap throughwhich said strip passes, one of said pole pieces being split indirection normal to the axis of vibration of said strip to provide legs,means for producing in said gap an alternating field at a frequencyhigher than those of said seismic waves, said field inducing eddycurrents in said metallic strip, and a pair of coils respectivelydisposed on said legs for induction of voltages therein by the magneticfields of said eddy currents, and means for connecting said coils inseries opposition to produce a joint output substantially proportionalto the velocity of said strip.

stages and including broadly resonant at frequency of source ReferencesCited in the file of this patent UNITED STATES PATENTS 2,104,131Matthews Jan. 4, 1938 2,130,213 Wolf Sept. 13, 1938 2,155,267 HathawayApr. 18, 1939 2,269,453 Gayhart Jan. 13, 1942 2,296,754 Wolf Sept. 22,1942 2,372,056 Broding Mar. 20, 1945 2,405,185 Benioff Aug. 6, 19462,469,137 Strong May 3, 1949

